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Derivation of EMU, DEMU and trailer unit letter designations

The meanings of EMU, DEMU and trailer unit single, two and three letter designations are a frequently raised topic. Below are tables explaining the origin of these abbreviations. However this information comes with an important caveat - it is difficult to find an authoritative source, including in official documentation, to establish and confirm the derivation of such codes. It is thought possible that some codes had more than one official origin or that the official designation changed over time. Furthermore it is known that railwaymen, enthusiasts and passengers alike had their own variations, some of which may even have crept into semi-official use. What we display below is believed to be the official or close to official meanings and is given in good faith. The Southern Electric Group and its contributors cannot be held responsible for errors and omissions. If you spot anything incorrect and can verify from an independent source your information, you are invited to contact the SEG Webmaster so that we can make this information as accurate as possible.

The commonly known form of unit designations was introduced by the Southern Railway from 1933 onwards in connection with the Brighton main line electrification scheme. Such designations were not applied retrospectively to existing D.C. suburban stock which were referred to merely by the number of their cars, e.g. "3".; Thus the use of the designation 3 Sub, although seen in publications, is strictly incorrect. When these units were augmented to 4 cars by addition of a car from former trailer units they were referred to as "4". It is believed the designation 4 Sub was only used with the introduction of new stock. The SR designation system was also used by the British Railways Southern Region until TOPS codes finally took over in the 1980s, although in defiance some residual use remained! Today, of all the SR area TOCs, the convention is only maintained on new units by South West Trains and possibly Gatwick Express.

You may have noticed that the format of such designations, including the preceding numeral, varies between communications and publications. The Southern Electric Group is conforming to the designations as decided by BR(S) headquarters, though it is recognised that in actual use by railwaymen and even in official communications the correct format and designations have not been used consistently. The BR(S) HQ official format is: numeral (numeral one not used), space (NOT a hyphen), single two or three letter code, full stop (period) if an abbreviation; e.g. 4 Sub., 2 EPB, MLV. As to whether the letter abbreviation is all capitals or mixed case there does not appear to be a consistent convention, and the case used is what was decided by officialdom at the time!

Designations are amended in operating documents where the make up of the train is to be specified. In particular the numeric element is changed to reflect the total number of cars of the unit type in the train. Thus a train consisting of two 4Cig units would be specified as 8Cig, a typical Bournemouth line train made up of Mk1 stock would be specified as 4Rep,8TC and for several years a morning peak up Bournemouth line semi-fast consisting of 8Vep and up Salisbury train consisting of 4TC,DL combined at Basingstoke to run to Waterloo as 4TC,DL,8Vep. Where, as with EPB stock, unit types could be 2 or 4 car the specification was only for the total length and not the specific make up. Thus 8EPB could in reality be two 4EPB, one 4EPB plus two 2EPB or even four 2EPB. Following TOPS class designations being adopted on the Southern Region a numeric prefix specifying the total length (not the number of units) is added to the class number. Therefore a service to be run as two class 455 units is specified as 8.455, a service of three class 456 units is specified as 6.456 and a combination of one class 455 and two class 456 is specified as 455,4.456.

This page is only about the origin of unit designations. For further details of the units mentioned please see our Rolling Stock Directory.   This feature is a co-production with the Southern E-Group.

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SR/BR(S) Code Derivation Note
B B uffet e.g. 6B, Hastings units.
D Oxte D e.g. 3D
H H ampshire and Sussex e.g. 2H
L L ong underframe e.g. 6L, Hastings units
R R eading and Redhill e.g. 3R, nicknamed Tadpole units.
S S hort underframe e.g. 6S, Hastings units.
T additional T railer e.g. 3T, origin of this derivation uncertain .  These were not Tadpole units but 2H units supplemented by a DTC from withdrawn 3R units.

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EMUs - ac
LBSCR Code Derivation Note
CP C rystal P alace LBSC AC stock.
CW C oulsdon and W allington LBSC AC stock.  Some sources query the origin of the W, and it has also been seen given as W andsworth.
SL S outh L ondon LBSC AC stock.

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EMUs - dc
SR/BR(S) Code Derivation Note
Bel. Brighton Bel le e.g. 5 Bel.
Bep. B uffet E lectro P neumatic Brake e.g. 4 Bep.
Big. B uffet IG and B uffet I ntermediate G uard e.g. 4 Big.  IG was the LBSCR telegraphic code for Brighton, on which line these units were first used.
Bil. Bi l avatory e.g. 2 Bil. A lavatory in each car.
Buf. Buf fet e.g. 4 Buf.
Cap. C oastway H ap e.g. 4 Cap.  Also see Hap.
Cep. C orridor E lectro P neumatic Brake e.g. 4 Cep.
Cig. C orridor IG and C orridor I ntermediate G uard e.g. 4 Cig. IG was the LBSCR telegraphic code for Brighton, on which line these units were first used.
Cit. Cit y e.g. 6 Cit.
Cop. C orridor O pen P lan e.g. 3 Cop.
Cor. Cor ridor e.g. 4 Cor.
DD D ouble D eck e.g. 4DD
Des. Des iro e.g. 4 Des.
Dig. D ouble IG e.g. 8 Dig. See Big/Cig.
EPB E lectro P neumatic  B rake e.g. 2 EPB
Gat. Gat wick Express e.g. 8 Gat.  This may not be an official designation.
GLV G uards L uggage V an i.e. class 489.
Gri. Gri ddle e.g. 4 Gri.
Hal. Ha lf l avatory e.g. 2 Hal.  One lavatory in the whole unit.
Hap. Ha lf lavatory with Electro P neumatic Brakes, i.e.  EPB version of Hal. e.g. 2 Hap.
Jop. J uniper o uter suburban units owned by
P orterbrook, though could be J uniper o pen p lan.
e.g. 4 Jop.  Query the official derivation?
Lav. Lav atory e.g. 4 Lav.
Mig. M icro buffet IG e.g. 8 Mig. See Big/Cig.
MLV M otor L uggage V an i.e. eventually class 419.
Nol. No l avatory e.g. 2 Nol.
Pan. Pan try e.g. 6 Pan.
Pan. P arcels a nd n ewpapers e.g. 2 Pan. 1971 re-use of designation but these were not the same 1930s main line units.  These were parcels units converted from Hals.
Pep. P rototype E lectro P neumatic Brake e.g. 4 Pep.
Pul. Pul lman e.g. 6 Pul.
Rep. R estaurant E lectro P neumatic Brake e.g. 4 Rep.
Res. Res taurant e.g. 4 Res.
Sap. S econd class only H ap e.g. 2 Sap. See Hap.
SL S outh L ondon e.g. 2 SL
Sub. Sub urban e.g. 4 Sub. See text in introduction re. incorrect use of 3 Sub.
TEP T emporary E lectro P neumatic Brake e.g. 4 TEP
Tis. Vec tis e.g. 3 Tis.
Vab. V ep a nd b uffet e.g. 8 Vab. See Vep.
Vec. Vec tis e.g. 4 Vec.
Veg. Ve p modified for G atwick Airport services e.g. 4 Veg. See Vep
Vep. V estibuled E lectro P neumatic Brake e.g. 4 Vep.
Vop. V estibuled o pen p lan e.g. 4 Vop.
Wes. Wes sex and W essex E lectric S tock e.g. 5 Wes. Believed to be an unofficial designation that has crept into semi-official use.
Wim. Wim bledon and West Croydon e.g. 2 Wim.

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Trailer Multiple Units
SR/BR(S) Code Derivation Note
TC T railer C ontrol set e.g. 4 TC
TCB TC with B uffet e.g. 5 TCB. See TC.
TLV T railer L uggage Van i.e. eventually class 499.